straightening out backend process startup

Hi,

I've previously complained ([1]) that process initialization has gotten
very complicated. I hit this once more last week when trying to commit
one of the shared memory stats pieces...

I think there's quite a few different issues around this - here I'm just
trying to tackle a few of the most glaring (to me):

- AuxiliaryProcessMain() is used for two independent tasks: Start bootstrap /
checker mode and starting auxiliary processes. In HEAD there's maybe 5 lines
out 250 that are actually common to both uses.

A related oddity is that we reserve shared memory resources for bootstrap &
checker aux processes, despite those never existing.

This is addressed in patches 1-7

- The order of invocation of InitProcess()/InitAuxiliaryProcess() and
BaseInit() depends on EXEC_BACKEND. Due to that there often is no single
place initialization code can be put if it needs any locks.

This is addressed in patches 8-9

- PostgresMain() has code for single user and multi user interleaved, making
it unnecessarily hard to understand what's going on.

This is addressed in patches 10

This isn't a patch series ready to commit, there's a bunch of polishing that
needs to be done if there's agreement.

Questions I have:

- What exactly to do with checker mode: Keep it as part of bootstrap, separate
it out completely? What commandline flags?

- I used a separate argv entry to pass the aux proc type - do we rather want
to go for the approach that e.g. bgworker went for? Adding code for string
splitting seems a bit unnecessary to me.

- PostgresMainSingle() should probably not be in postgres.c. We could put it
into postinit.c or ..?

- My first attempt at PostgresMainSingle() separated the single/multi user
cases a bit more than the code right now, by having a PostgresMainCommon()
which was called by PostgresMainSingle(), PostgresMain(). *Common only
started with the MessageContext allocation, which did have the advantage of
splitting out a few of the remaining conditional actions in PostgresMain()
(PostmasterContext, welcome banner, Log_disconnections). But lead to a bit
more duplication. I don't really have an opinion on what's better.

- I had to move the PgStartTime computation to a bit earlier for single user
mode. That seems to make sense to me anyway, given that postmaster does so
fairly early too.

Any reason that'd be a bad idea?

Arguably it should even be a tad earlier to be symmetric.

There's one further issue that I think is big enough to be worth
tackling in the near future: Too many things depend on BackendIds.

Aux processes need procsignal and backend status reporting, which use
BackendId for indexing. But they don't use sinval, so they don't have a
BackendId - so we have hacks to work around that in a few places. If we
instead make those places use pgprocno for indexing the whole issue
vanishes.

In fact, I think there's a good argument to be made that we should
entirely remove the concept of BackendIds and just use pgprocnos. We
have a fair number of places like SignalVirtualTransaction() that need
to search the procarray just to find the proc to signal based on the
BackendId. If we used pgprocno instead, that'd not be needed.

But perhaps that's a separate thread.

Greetings,

Andres Freund

[1] https://postgr.es/m/20210402002240.56cuz3uo3alnqwae%40alap3.anarazel.de